Intercalation-driven ferroelectric-to-ferroelastic conversion in a layered hybrid perovskite crystal

Two-dimensional (2D) organic-inorganic hybrid perovskites have attracted intense interests due to their quantum well structure and tunable excitonic properties. As an alternative to the well-studied divalent metal hybrid perovskite based on Pb2+, Sn2+ and Cu2+, the trivalent metal-based (eg. Sb3+ with ns2 outer-shell electronic configuration) hybrid perovskite with the A3M2X9 formula (A = monovalent cations, M = trivalent metal, X = halide) offer intriguing possibilities for engineering ferroic properties. Here, we synthesized 2D ferroelectric hybrid perovskite (TMA)3Sb2Cl9 with measurable in-plane and out-of-plane polarization. Interestingly, (TMA)3Sb2Cl9 can be intercalated with FeCl4 ions to form a ferroelastic and piezoelectric single crystal, (TMA)4-Fe(iii)Cl4-Sb2Cl9. Density functional theory calculations were carried out to investigate the unusual mechanism of ferroelectric-ferroelastic crossover in these crystals.


REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): In this work, Wu et al report the synthesis of a ferroelectric hybrid perovskite TSC, and by FeCl4intercalation they observed a transition of ferroelectric-to-ferroelastic, which is due to the intercalation induced rotation of the localized lattice units and thereby loss of ferroelectricity. The intercalation methods and likewise the resultant effect is of certain value to tune the ferroelectricity of these 2D halide perovskite materials. However, the TSC seems to be a material that has been studied before. Here are several reports studying similar composition of the A3Sb2X9: e.g., Journal of Physics: Condensed Matter, 2003, 15, 5765.;The Journal of Physical Chemistry A 2005, 109, 3097-3104.;Acta Crystallogr. Sect. B Struct. Sci. 1996, 52, 287-295. Taking the Bi3+ based 2d halide perovskite, the trivalent metal-centered 2D hybrid perovskites have already been known as candidate for ferroelectric materials. From this view, the main novelty I understand of this work is the FeCl4-intercalation methods that modulate the lattice and the consequent leading to changes of signature properties. Nevertheless, they provide a comprehensive work on how to use intercalation to modulate the materials property at a perspective of lattice and also provide calculations to explain along with this strategy, this work is of interest.
There are a few detailed technical comments: • Line 83 "Single crystal XRD of TSFC indicates", I didn't see single crystal XRD data.
• The intercalation causes the lattice strain and in-plane unit cell expansion, this is the result of structure refinement analysis and observation from the experiment. What is the atomic reason for this strain? Atomic size mismatch? Or coulomb interaction or other spin-related effect? It would be great if the authors could shed some light on this.
• It seems the bandgap shrinks upon the intercalation, will this induce any leakage effect (making it more semiconductive) to the ferroelectricity measurement?
• It is suggested to add quantitive analytical plot on e.g., Fig. 5d-f, it is hard to read from the image of the 0.2 um changes.
Reviewer #2 (Remarks to the Author): In this work, the authors report the first intercalation-induced unusual ferroic order transition from the ferroelectric TSC to ferroelastic TSFC in the 2D layered perovskite, while keeping the piezoelectricity. The ferroelectricity of TSC was confirmed by P-E loops and PFM, and the ferroelastic domain motions in TSFC were also observed by applying external strain. Furthermore, density functional theory calculations were performed to understand the basis of ferroelectricity and ferroelasticity in TSC and TSFC. This work is interesting and important, and conclusions are well supported by experimental results. Thus, I would like to recommend publication of this work in Nature Communications after a minor revision: 1. Accompanying the paraelectric-to-ferroelectric phase transition, the paraelectric and ferroelectric phase have group-to-subgroup relationship determined by the Curie principle. This enables one to judge uniaxial or multiaxial ferroelectric by evaluating the number of equivalent polarization directions. Thus, single-crystal structure in high temperature paraelectric phase should be provided to expound "authors claim that TSC is the uniaxial ferroelectric".
2. Curie temperature Tc is one of important parameters for ferroelectric material. The authors should provide some experimental results (such as DSC) to uncover the Tc of TSC.
3. The experimental description on piezoelectricity measurement by laser scanning vibrometer method needs much more details. Figure 3j is too small to distinguish a and b-axis.
Reviewer #3 (Remarks to the Author): The manuscript by Wu et al reported the synthesis of a ferroelectric layered perovskite derivative (TMA)3Sb2Cl9 and a ferroelastic (TMA)4-FeCl4-Sb2Cl9 by intercalating (TMA)FeCl4 into the layer space of (TMA)3Sb2Cl9. The single-crystal crystal structures along with ferroelectric, ferroelastic, and piezoelectric properties were well-characterized. The authors performed symmetry analysis on the two structures to understand the absence of ferroelectricity after intercalation. They also carried out DFT calculations to understand atomistic structures of ferroelastic domains in (TMA)4-FeCl4-Sb2Cl9. Overall, the scientific methods and results are sound. I personally feel the intercalation chemistry very interesting, which provide a new chemical strategy to tune ferroic properties of perovskite-like structures. However, I have a hard time recommending it for publication in Nature Communications based on the two following concerns.
First, while I personally feel the intercalation chemistry interesting, I do not find the presented results in the current form of particularly appealing. The authors spent a large body of discussion on the characterization of ferroelastic properties of (TMA)4-FeCl4-Sb2Cl9. However, the properties of this particular compound are not all that fascinating. I am not certain that these properties would be of potential to enable disruptive applications. Based on my research background and interests, I am more curious to know more about the intercalation chemistry and its influences on the properties. For example, can the intercalation chemistry apply to other compounds or just this compound? Can the ferroic properties can be rationally tuned by the intercalating species? etc. The information is currently missing.
Second, the authors claimed in the abstract and introduction that the perovskite-like structure with the A3M2X9 formula is a less explored system for ferroic properties. This is not correct. (2) (C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9: ferroelastic lead-free hybrid perovskite-like materials as potential semiconducting absorbers, Dalton Trans., 2022,51, 1850-1860; These works are not appropriately acknowledged. The authors should also compare their results with these previous works and highlight their novelty.
Minor comments 1. Do the authors have the information on the phase evolution with temperature for the two structures? 2. On page 5, line 172, the authors state that "The in-plane 3.1% expansion of unit cells in TSFC relative to TSC creates strain axes of C3V symmetry and enables ferroelasticity". Do the authors infer the absence of ferroelasticity in (TMA)3Sb2Cl9?

Response to reviewers
Reviewer #1 (Remarks to the Author): In this work, Wu et al report the synthesis of a ferroelectric hybrid perovskite TSC, and by FeCl4 − intercalation they observed a transition of ferroelectric-to-ferroelastic, which is due to the intercalation induced rotation of the localized lattice units and thereby loss of ferroelectricity. The intercalation methods and likewise the resultant effect is of certain value to tune the ferroelectricity of these 2D halide perovskite materials. However, the TSC seems to be a material that has been studied before. Here are several reports studying similar composition of the A3Sb2X9: e.g., Journal halide perovskite, the trivalent metal-centered 2D hybrid perovskites have already been known as candidate for ferroelectric materials. From this view, the main novelty I understand of this work is the FeCl4 − intercalation methods that modulate the lattice and the consequent leading to changes of signature properties. Nevertheless, they provide a comprehensive work on how to use intercalation to modulate the materials property at a perspective of lattice and also provide calculations to explain along with this strategy, this work is of interest.
> We thank the reviewer for the recognition of the novelty and great support to the manuscript.
Some references reviewer mentioned have been added to enrich our manuscript.
There are a few detailed technical comments:  Line 83 "Single crystal XRD of TSFC indicates", I didn't see single crystal XRD data. > Response: Thanks for your useful suggestion. When Fe-Cl tetrahedron is inserted, the structural framework of the inorganic layer remains unchanged, but the arrangement of the adjacent layer changes from the parallel arrangement of the original adjacent layers to the antiparallel arrangement with 21 helical symmetry. This results in a significant increase in the spacing between layers because of Fe-Cl tetrahedron with large steric hindrance. Meanwhile, Fe-Cl tetrahedron is located in the center of the upper and lower six-membered rings composed of Sb-Cl octahedra. Since the two inorganic components of Fe-Cl tetrahedron and Sb-Cl ring are both negatively charged, their mutually repulsive Coulomb interaction leads to the expansion of six-membered rings. This results in the in-plane unit cell expansion. In other words, we believe that the size mismatch and coulomb interaction is the main causes of strain in the structure.
 It seems the bandgap shrinks upon the intercalation, will this induce any leakage effect (making it more semiconductive) to the ferroelectricity measurement? > Response: We use positive-up-negative-down (PUND) method to perform the P-V measurements. As shown in the following figure, the obvious leakage effect of TSFC for ferroelectricity measurement can be observed. Generally, for a ferroelectric material, the P-V hysteresis loop consists of three components: ferroelectric, dielectric, and leakage components in the P process. However, during the U processes, measured curve mainly includes dielectric and leakage components. These non-ferroelectric contributions were deducted by subtracting the U data from the P ones. Finally, a standard P-V hysteresis loop can be obtained. For intercalated perovskite TSFC, the positive branches of P and U curves are almost exactly the same, indicating the non-ferroelectric nature of TSFC. We only show the remanent curves of TSFC in the supporting information (Figure 13 a).
Positive branches of the P-V hysteresis loops measured by a positive-up negative-down (PUND) method.
 It is suggested to add quantitive analytical plot on e.g., Fig. 5d-f, it is hard to read from the image of the 0.2 um changes.
> Response: Thanks for your useful advice. We have added line profile analysis of the domain width in Fig. 5d, 5e and 5f. The amplitude signals along lines show a tip-orientation at ferroelastic domain walls, resulting from a strain gradient leading to the local structure piezoelectrically active (Nature communicaitons, 2020, 11, 4898). We believe that it is now easy to read the ferroelastic domain changes from the revised images. In this work, the authors report the first intercalation-induced unusual ferroic order transition from the ferroelectric TSC to ferroelastic TSFC in the 2D layered perovskite, while keeping the piezoelectricity. The ferroelectricity of TSC was confirmed by P-E loops and PFM, and the ferroelastic domain motions in TSFC were also observed by applying external strain. Furthermore, density functional theory calculations were performed to understand the basis of ferroelectricity and ferroelasticity in TSC and TSFC. This work is interesting and important, and conclusions are well supported by experimental results. Thus, I would like to recommend publication of this work in Nature Communications after a minor revision:  Response: We thank the reviewer for his or her positive comments.
1. Accompanying the paraelectric-to-ferroelectric phase transition, the paraelectric and ferroelectric phase have group-to-subgroup relationship determined by the Curie principle. This enables one to judge uniaxial or multiaxial ferroelectric by evaluating the number of equivalent polarization directions. Thus, single-crystal structure in high temperature paraelectric phase should be provided to expound "authors claim that TSC is the uniaxial ferroelectric".   First, while I personally feel the intercalation chemistry interesting, I do not find the presented results in the current form of particularly appealing. The authors spent a large body of discussion on the characterization of ferroelastic properties of (TMA)4-FeCl4-Sb2Cl9. However, the properties of this particular compound are not all that fascinating. I am not certain that these properties would be of potential to enable disruptive applications. Based on my research background and interests, I am more curious to know more about the intercalation chemistry and its influences on the properties.
For example, can the intercalation chemistry apply to other compounds or just this compound? Can the ferroic properties can be rationally tuned by the intercalating species? etc. The information is currently missing.  Response: Thanks for review's suggestion. We agree with the reviewer that the intercalation chemistry is interesting because it can modify ferroic properties. Recently, two-dimensional hybrid organic-inorganic perovskites have emerged as a new class of optoelectronic and ferroic materials with the benefits of easy processing, structural diversity, mechanical flexibility, and intrinsic quantum-well effects. Therefore, by taking advantage of the layered structure that allows intercalation by molecules, ions or atoms, some novel properties may appear, which allows the engineering of new ferroic properties. Here, our work is focused on uncovering fundamental aspect of structure chemistry and admittedly at this stage we did not show any disruptive applications. However, intercalation chemistry may allow us to uncover multiferroic properties and more research is needed.
The reason why we dedicate quite a bit of length to present data ferroelasticity is because it is non-trivial to prove that the material is ferroelastic, as stripe-like domains that can be seen on the surfaces of these crystals are often confused with ferroelectricity. To provide rigorous proof, that the crystal has changed from ferroelectric to ferroelastic, we need to perform the described experiments and provide the theoretical basis for it.
In terms of application, Yi et al. reported the anomalous photovoltaic effect in centrosymmetric ferroelastic BiVO4, where photovoltaic voltage can be reversed by stress modulation (Adv. Mater., 2018, 30, 1870334). This is assigned to the flexoelectric coupling via a strain-induced local polarization mechanism. Here, we study the ferroelastic domain structure and atomistic arrangement for different domains, which provide the foundation of future flexoelectric memory devices. In addition, this compound also has excellent piezoelectric performance comparable to inorganic piezoelectric materials.
In addition, we have also tested the intercalation concept on some other two-dimensional (2D) hybrid intercalation perovskites. One example of which is 2D intercalation perovskite (pyrrolidinium)4-FeCl4-Sb2Cl9 (as shown in the following picture). Without intercalation, the ferroelasticity of the host compound will be lost above 241 K, whereas intercalation of the perovskite stabilizes the ferroelasticity at room temperature. In summary, we think our work provides a new avenue to tune the ferroic properties of layered perovskite by intercalation chemistry, and pave the way forward for constructing multiferroic materials. > Response: Thanks for these useful suggestions, the literatures mentioned by the reviewer have been cited in the revised manuscript.
Perovskite-like structure in trivalent metal-based (Sb 3+ ,Bi 3+ ) with A3M2X9 formula can be divided into four types: one-dimensional (1D) zig-zag double chains; two-dimensional (2D) layers; discrete bioctahedra (0D); four octahedral units (0D) ([M4X18]6−). Within the chemical stoichiometry A3M2X9 the ferroic properties are mainly manifested by two types of anionic sublayers: 2D and 0D-discrete bioctahedral units. The crystal structure in the literature 2, 3 and 4 adopts 0D-discrete bioctahedra framework and is not 2D. Only the compound in 1 belongs to 2D layered perovskite structure, but the fly in the ointment is that its P-E hysteresis loop could only be obtained at low temperature (less than 241 K). The standard P-E hysteresis loop of the compound TSC reported in this work can be obtained from 293 K to 323 K (Curie temperature ~ 363 K). Compared with well-studied divalent metal hybrid perovskite based on Pb 2+ , Sn 2+ and Cu 2+ , trivalent metal-based (Sb 3+ , Bi 3+ ) 2D perovskite systems with A3M2X9 formula were typically stabilized with simple and small organic cations (so far, pyrrolidinium is the largest one), which limits the scope for attaining ferroic properties. Therefore, it is timely to explore ways to tune ferroic properties in trivalent metal-based 2D perovskite system. Here, by using the intercalation chemistry method, we have successfully constructed 2D intercalation perovskite, and demonstrated that the ferroic order transforms form ferroelectricity to ferroelasticity. This work not only enhances the trivalent metal-based 2D perovskite system, but also provides a new way to construct ferroic materials and further to get the multiferroic materials.
To make our motivations clearer, the description of 2D A3M2X9 perovskites in the abstract and introduction have been revised (see the highlighted sections).
Minor comments 1. Do the authors have the information on the phase evolution with temperature for the two structures?
> Response: We have performed DSC to study the phase change of these two compounds.
DSC curves reveal TSC and TSFC have structure phase transition at 363 K and 325 K, respectively. TSC is the ferroelectric phase at room temperature and crystallizes in Pc space group and point group of m; while at the high temperature paraelectric phase, the space group changes from Pc to P21/c. According to Aizu notion, the symmetry breaking of TSC is 2/mFm, pertaining to the 88 potential ferroelectric phase transitions. For ferroelastic TSFC, it belongs to